1. Field of the Invention
This invention pertains to air-to-ground tracking and guidance systems and more particularly to an air-to-ground tracking and guidance system utilizing an illuminator/tracker and a tri-mode terminally guided seeker.
2. Description of the Prior Art
Previous air-to-ground terminal guidance seekers principally used autonomous dual mode (active/passive) seekers for weapons delivered in either a ballistic trajectory delivery or in a near horizontal trajectory. These systems were subsequently improved to include range measurement. A dual mode seeker system which incorporates this improvement is described by Lazarchik et al in U.S. Pat. No. 3,921,169 for a Multi-Mode Radiometric System with Range Detection Capability, issued Nov. 18, 1975 and assigned to Sperry Rand Corporation. The tracking capabilities of these systems have been enhanced with improvements made in signal-to-clutter ratio and signal-to-receiver noise ratio by employing techniques such as linear FMCW and multiple narrow frequency range bins for automatic search acquisition and range and angle tracking. A system employing the narrow frequency range bin concept is described by Roeder et al. in U.S. Pat. No. 4,200,871, Acquisition System for Continuous Wave Modulation Object Detector.
Semi-active or bistatic systems have been extensively employed in two major areas: laser designation systems and surface-to-air radar missile systems. These systems incorporate an illuminator or designator which can single out a target and illuminate it with a signal which the seeker can identify and use for homing. The laser systems success is attributable to the directivity of the designating beam which permits laser energy to be concentrated within a small area to which the homing device is directed. Since laser systems operate with optical beams, they lose effectiveness when fog or smoke obscures the line of sight between the observer and the target.
Semi-active surface-to-air radar systems generally operate against airborne targets which are isolated in space. This condition allows a homing seeker to be accurately directed to the target, though the coverage area of the illuminating beam is appreciably broader than the extent of the target.
Semi-active homing has not been heretofore employed against ground targets because of the difficulty of confining the illuminating beam to a single target. The addition of active/passive autonomous homing to the initial semi-active homing as described herein provides an over-all homing system with appreciable capability for air-to-ground systems. This addition provides for the transition from semi-active to active to passive operation as the seeker approaches the target. Since the passive mode which normally seeks the centroid of the radiating area is employed to the final mission, a high accuracy long range system is realized.